The corporation's Director General Mark Thompson said the BBC was still seeking clarification of the reporter's condition.

He praised Mr Johnston's family, calling the wait for news "agonising" for them.

On Sunday the Tawhid and Jihad brigades (The Brigades of Holy War and Unity), a previously unheard of group, said it had killed Mr Johnston, linking his purported killing to the plight of Palestinian prisoners held in Israel.

A spokesman for UK Prime Minister Tony Blair said the government was investigating the reports.

"We are working closely with the Palestinian Authority and are urgently seeking information from them and other sources," he said.

'Solidarity'

In the latest of almost daily demonstrations in the wake of Mr Johnston's abduction, journalists gathered outside the Lebanese Press Syndicate building in Beirut to press for his release.

"We would like to appeal today to whoever is holding [Alan Johnston] to release him today unharmed," the BBC's Beirut correspondent, Kim Ghattas, told media.

Our correspondent said Mr Johnston had continued to do his job professionally, despite the risks.

But, more than a month since his abduction, there was concern about Mr Johnston's mental health and physical state, she said.

Another demonstrator, Lebanese TV reporter Diana Moukalled, said it was very important "to show our solidarity with any colleague that's harmed because of his job.

"Because we, ourselves, have suffered a lot in Lebanon, we have a long history of targeting journalists, some of them were killed, others kidnapped, some of them were tortured.

"It's unfortunate for journalists to be targeted just because they are doing their job."